CMV infection causes lupin leaves to become pale, bunched and down curled with faint mosaic. In plants that grow from infected seed, all leaves develop these symptoms and these plants are severely stunted. Healthy plants that become infected during the growing season have pale, bunched young leaves with faint mosaic while older leaves formed prior to infection appear normal. As growth continues, all new leaves show symptoms and the infected plants become stunted. The earlier a plant becomes infected, the fewer the pods set, the smaller the size of seed produced and the smaller the yield. With late infections symptoms are restricted to tip leaves.
Sources of virus
Sowing infected seed produces infected seedlings scattered at random within the crop. Aphids pick up the virus from the infected plants and spread it to nearby healthy plants. When infection incidence reaches 100%, the maximum possible seed transmission rate to seedlings varies with lupin variety.
In the wheatbelt, although infected lupin seed is the only CMV source of consequence for lupin crops, other legume hosts can become infected, including clovers, chickpea, faba bean, lentil and field pea. Weeds that sometimes become infected include capeweed, stagger weed and fumitory, but the virus is not seed-borne in these species so infection is lost over summer.
CMV is spread by many aphid species, including green peach (Myzus persicae), blue green (Acyrthosiphon kondoi) and cowpea (Aphis craccivora) aphids that colonise lupins as well as migrants of common non-lupin colonising species, especially oat (Rhopalosiphum padi) and turnip (Lipaphis erysime) aphids. CMV is transmitted non-persistently: an aphid picks it up within one to two seconds while probing an infected plant, but the virus is then lost again when the aphid probes one or two healthy plants.